Even if coloration in fungi is an unanswered question at this point, how on earth is that an argument against the overall theory? All current theories have unanswered questions about details (otherwise scientists would be unemployed).

If I'm following this, the color itself doesn't appear to be the controlling factor. But, it's caused by chemicals that the organism produces and concentrates in its outer layers? The chemicals in question happen to be colorful, but has anybody checked on whether those chemicals produce some other benefit that maybe has nothing to do with their reaction to light?

That would after all seem to be the next question if mimicry and/or camoulflage have been determined to be unlikely.